Different look of material with Mix Shader vs Mix RGB
Mix shader use a very simple method to combine two shader output. It doesn't guarantee any real material based on physic light reproduce. You are actually using alpha compositing rather than reevaluate a BSDF shader. See the example below.



On the left we use Mix Shader to mix the Roughness between 1.0 and 0.0. Right we use real 0.5 value for a single Glossy BSDF.

You can see that the Glossy 100% reflection just blend into the rough one without any transition. This is totally impossible in real world unless a clear-coat is presented.

So if a single BSDF shader can do the job, don't try to mix other shader, just let it do the calculation and connect it to the output directly.